Plasma torch for microwave induced plasmas

ABSTRACT

A Plasma torch ( 10 ) for microwave induced plasma spectrochemical analysis of a sample includes a nozzle ( 30 ) in an inlet ( 18 ) for the main plasma gas flow between outer tube ( 12 ) and intermediate tube ( 14 ) of the torch ( 10 ). The nozzle ( 30 ) increases the gas flow velocity in the sheathing gas layer for the plasma which is provided by the gas flow from the annular gap ( 22 ) between the tubes ( 12  and  14 ). The increased velocity of the gas in the sheathing gas layer “stiffens” that layer and thus better confines the microwave induced plasma (such better confinement not being necessary for an ICP torch). Thus the torch is of improved durability for a microwave induced plasma compared to an ICP torch. The sample injection (inner) tube ( 16 ) may have a reduced diameter outlet at its end ( 34 ) which is substantially level with the end ( 35 ) of intermediate tube ( 14 ) to improve injection of a sample into the microwave induced plasma. The inlet end ( 26 ) of the sample injection tube ( 16 ) may include a heater ( 36 ) to assist in preventing blockages in tube ( 16 ) near its outlet end.

TECHNICAL FIELD

The present invention relates to a torch for plasma spectrochemicalanalysis, in particular a microwave induced plasma (MIP)torch.

BACKGROUND

It is known that a plasma for spectrochemical analysis, for example forthe elemental analysis of liquid samples, can be electrically excited,for example with radio frequency energy or microwave energy. Plasmasthat are excited by radio frequency energy, that is, inductively coupledplasmas (ICP), are now well developed. In ICP spectrometry, the plasmais formed in a torch by induction from a surrounding coil excited withradio frequency energy, typically at between 20 and 50 MHz. The plasmaforms as a hollow cylinder allowing injection of sample into the hollowcentral core of the plasma. Acceptable performance of ICP spectrometryrequires close control of the gas flow regime including a sheathing gasflow around the plasma. In a typical ICP torch, regulation of the gasflows is ensured by a separate and independent gas control system, andthe gas inlets into the torch are large relative to the amount of gasbeing admitted such that the presence of the torch creates very littleback pressure.

Microwave induced plasma (MIP) spectrometry, however, is less welldeveloped than ICP spectrometry, despite offering advantages, forexample the availability of low cost, rugged and reliable microwavegenerators in the form of magnetrons. This is because the analyticalperformance of MIP systems has, until a recent development of theapplicant, been significantly inferior to ICP systems. In theapplicant's recently developed MIP system, a plasma torch is locatedwithin a microwave cavity for either the magnetic field component orboth the magnetic and electric field components of the microwave energyto excite a plasma in the torch. A plasma having a tubular form ofelliptical cross-section can be formed in the torch and the system hasshown analytically useful performance approaching that obtainable withradio frequency ICP systems.

The inferior performance of MIP systems is due in large measure to themicrowave induced plasma having different characteristics to a radiofrequency ICP. Thus in a microwave induced plasma, the plasma thicknessis much smaller and has a smaller core region compared to a radiofrequency plasma (a microwave plasma exhibits substantially highertemperature vs. position gradients across a torch compared to a radiofrequency ICP). These characteristics of a microwave induced plasma makethe plasma more difficult to confine such that a torch as usually usedfor ICP spectrometry is generally not suitable for MIP spectrometry.

The discussion herein of the background to the invention is included toexplain the context of the invention. This is not to be taken as anadmission that any of the material referred to was part of the commongeneral knowledge in Australia as at the priority date of any of theclaims.

SUMMARY OF THE INVENTION

The present invention seeks to provide a microwave induced plasma torchfor spectrochemical analysis.

According to the invention there is provided a microwave induced plasmatorch for spectrochemical analysis including

-   -   an outer tube, an intermediate tube and an inner tube, the inner        tube being substantially coaxially located within the        intermediate tube for injecting a first gas flow for carrying a        sample for analysis into a microwave induced plasma produced in        the torch,    -   an intermediate-gas inlet leading into the intermediate tube for        admitting a second gas flow into the space between the inner        tube and the intermediate tube for controlling the axial        position of a microwave induced plasma produced in the torch,    -   an outer-gas inlet leading into the outer tube for supplying a        third gas flow between the outer tube and the intermediate tube        for providing a sheathing gas layer for a microwave induced        plasma produced in the torch,    -   wherein the outer-gas inlet is offset from a central axis of the        torch to impart a spiral flow to the supplied third gas as it        moves along the torch to provide the sheathing gas layer,    -   and a restriction within the outer-gas inlet for increasing the        gas velocity in the sheathing gas compared to the gas velocity        upstream of said restriction to thereby increase the confining        force of the sheathing gas layer on the microwave induced        plasma, the restriction providing for flow rate regulation from        a substantially constant pressure supply of the third gas.

In use, the increase in gas velocity creates a pressure drop across saidrestriction within the outer-gas inlet. Preferably the restriction hasan orifice having a cross sectional area which is such that, relative tothe cross sectional area of the outer gas inlet prior to the restrictionand for a given third gas, a pressure reduction of between 50 to 200 kPain the third gas when supplied to the outer gas inlet occurs across therestriction.

The increased velocity of the gas in the sheathing gas layer effectively“stiffens” that layer and thus better confines a microwave inducedplasma. This sheathing gas layer provides a boundary layer of gasbetween the inner surface of the outer tube of the torch and themicrowave induced plasma and thus keeps the plasma separated from thattube to prevent the tube from melting thereby improving the durabilityof the torch. The outer-gas inlet is located such that the direction ofgas flow at the point of injection of the gas flow is offset from thecentre line of the torch whereby the sheathing gas layer spins as itmoves along the length of the torch. This rotation, that is, spirallingof the gas flow helps to stabilise the plasma and maintain its uniformtubular form.

The increase in gas velocity is preferably relatively high such that therate of rotation of the gas sheathing layer is increased. Therestriction for increasing the gas velocity acts to convert thepotential energy inherent in the supply gas pressure to kinetic energywhere the gas enters the torch. Consequently, for a relatively highincrease in gas velocity in use, a significant pressure reductionoccurs. This is done proximate to where the gas enters the torchotherwise the kinetic energy would be dissipated through turbulence inthe tubing between the gas supply and the torch.

Preferably the restriction within the outer-gas inlet is a nozzle andthis may be a venturi or of a more complex shape to deliver betterenergy conversion efficiency.

The pressure reduction due to the presence of the velocity increasingrestriction associated with the outer-gas inlet exhibits a substantialif not dominant effect on regulation of the third gas flow to themicrowave induced plasma, that is, the torch constitutes a majorcomponent in the regulation of the third gas flow to the microwaveinduced plasma. This is opposite to the situation in a typical ICPsystem, wherein the gas flow to the plasma is supplied to the torch by acontrol system designed to provide a constant flow rate and in which thetorch has a negligible effect on the regulation of the gas flow. Thusthe invention makes it possible to supply gas to the torch at constantpressure rather than constant flow rate, and to rely on the torch forflow regulation.

Accordingly the invention also provides a microwave induced plasmaspectrochemical analysis system including

-   -   a microwave induced plasma torch as described hereinbefore,    -   a gas supply for supplying a plasma support gas to the outer-gas        inlet of the torch,    -   wherein the gas supply supplies the plasma support gas at a        substantially constant pressure,    -   whereby the flow rate of the third gas into the torch is        regulated by the restriction within the outer-gas inlet for        increasing the gas velocity in the sheathing gas layer.

As in a radio frequency ICP system, the microwave induced plasma torchincludes an inner tube for injecting a sample for spectrochemicalanalysis into the core of the plasma. Such an inner tube is normallylocated substantially coaxially within the intermediate tube. It is moredifficult to inject a sample into a microwave induced plasma than into aradio frequency plasma and to reduce this difficulty, the inner tube ofa torch according to an embodiment of the invention may have a reduceddiameter opening at its outlet tip. For example, whereas the preferredoutlet opening for a radio frequency ICP torch is between about 1.4 mmand 2.5 mm for aqueous samples, for a torch for a microwave inducedplasma using the same sample gas flow of about 1 litre per minute, theopening diameter may be between 0.9 and 1.4 mm. Additionally oralternatively, the outlet end of the inner tube may be extended to becloser to the microwave induced plasma than is typically the case for aradio frequency ICP torch. This means that the gas jet that containssample will have less distance to bend or diffuse before encounteringthe microwave induced plasma. In a preferred embodiment of theinvention, the outlet end of the inner tube is made substantially levelwith the end of the intermediate tube.

Another problem encountered in torches for both ICP and MIP,particularly for samples that contain high total dissolved solids (TDS),is that radiated energy from the plasma heats up the outlet end of theinner (that is, the sample injection) tube and can lead to blockage ofthat tube. That is, a small portion of the liquid droplets in anebulised sample travelling through the sample injection tube inevitablycontact the inner surface of the tube and tend to adhere thereto and aredried by the heated tube. The solid component of such droplets remainsattached to the inner surface and this deposit slowly builds upprogressively occluding the inner (sample injection) tube near or at itsoutlet opening. The effect is a slowly degrading signal, with thesensitivity becoming progressively worse. This is particularly a problemfor a microwave induced plasma torch if the sample injection (inner)tube is extended to be closer to the microwave induced plasma and/or hasa relatively smaller outlet opening, as described hereinbefore.

Another aspect of the invention seeks to avoid or at least reduce thisblockage problem when aspirating samples containing high TDS.

Accordingly the invention furthermore provides

-   -   a torch for plasma spectrochemical analysis including    -   an outer tube, an intermediate tube and an inner tube, the inner        tube being substantially coaxially located within the        intermediate tube for carrying a first gas flow for conveying an        aerosol of a nebulised sample liquid for injection through an        outlet thereof into a plasma formed in the torch,    -   an intermediate-gas inlet leading into the intermediate tube for        admitting a second gas flow into the space between the inner        tube and the intermediate tube for controlling the axial        position of a plasma produced in the torch,    -   an outer-gas inlet leading into the outer tube for supplying a        third gas flow-between the outer tube and the intermediate tube        for providing a sheathing gas layer for a plasma produced in the        torch,    -   wherein the outer-gas inlet is offset from a central axis of the        torch to impart a spiral flow to the supplied third gas as it        moves along the torch to provide the sheathing gas layer,    -   and a heater associated with a section of the inner tube for        heating an aerosol passing through that section to substantially        completely evaporate liquid from the aerosol, the section of the        inner tube being spaced from the outlet of the inner tube for        the liquid to be substantially completely evaporated before the        aerosol reaches the proximity of the outlet.

It should be noted that while one possibility is for the water to beremoved (that is, the sample desolvated) this is not a necessaryrequirement for the aspect of the invention disclosed immediately above.It is only necessary that the water be kept in gaseous form.

The heater may be a part of the torch as such or it may be otherwiseassociated with the torch, that is, the heater may be located along asection of the sample inlet tube between the output of the spray chamberand the sample inlet port of the torch. The heater preheats thenebulised sample aerosol to evaporate its liquid phase leaving dryparticles of sample suspended in the gas stream. If such dry particlescontact the wall of the injection (that is, the inner) tube, they slideover that wall without adhering thereto thus avoiding or at leastreducing the blockage problem.

Preferably a heater of a torch according to the “another aspect” of theinvention as described, hereinbefore is included with a microwaveinduced plasma torch of the aspect of the invention as first describedhereinbefore.

For a better understanding of the invention and to show how the same maybe carried into effect, a preferred embodiment thereof will now beaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates a preferred embodiment of a microwaveinduced plasma torch according to the invention.

FIGS. 2A, B and C illustrate steps for forming a nozzle in the gas inletof an embodiment of a microwave induced plasma torch according to theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

A microwave induced plasma torch 10 according to an embodiment of theinvention comprises three concentric tubes, typically of quartz, namelyan outer tube 12, an intermediate tube 14 and an inner tube 16. Theouter tube 12 includes an outer-gas inlet 18 for supplying a gas flow(hereinbefore “a third gas flow”) between the outer tube 12 and theintermediate tube 14. The intermediate tube 14 has an end section 20which together with the outer tube 12 defines an annular gap 22 forpassage of the third gas. The third gas flow between the outer andintermediate tubes 12 and 14 (termed the main flow or plasma support gasflow) establishes a sheathing gas layer for a microwave induced plasmaproduced in the torch which separates the microwave induced plasma fromthe inner surface of the quartz outer tube 12 and thus stops this tubefrom melting. The outer-gas inlet 18 is arranged for the gas to beinjected offset from the centre line of the torch such that the flowspirals or spins as it moves along the length of the microwave inducedplasma torch 10. This spiral flow of the gas sheath helps to stabilisethe microwave induced plasma and maintain its uniform tubular form. Theannular gap 22 is such as to help to maintain the sheathing gas layer asa thin laminar flow bordering the inner wall of the outer tube 12. Theend section 20 of the intermediate tube 14 may be of enlarged diameter(not shown) compared to the remainder of the tube 14 to define a smallerannular gap 22.

Intermediate tube 14 includes an intermediate-gas inlet 24 for supplyinga second gas flow between the intermediate tube 14 and inner tube 16.This flow is used to control the axial position of the microwave inducedplasma and in particular to keep it separated from the ends 35 and 34respectively of the intermediate tube 14 and inner tube 16.

The inner tube 16 is for containing a flow of gas (hereinbefore “a firstgas flow”) for carrying sample aerosol supplied to its inlet end 26 andinjects this into the core of the microwave induced plasma. This tube 16may include a gradual taper 28 along a substantial portion of its lengthto improve the torch performance as disclosed in the applicant's priorapplication No. PCT/AU02/00386 (WO 03/005780 A1) entitled “PlasmaTorch”.

For excitation of a microwave induced plasma, torch 10 would be suitablyassociated with means for applying a microwave electromagnetic field tothe torch, for example, torch 10 may be appropriately located through aresonant cavity to which microwave energy is supplied. A plasma may beinitiated by momentarily applying a high voltage spark (by means knownin the art and not shown) to the gas entering through inlet 18.

According to an aspect of the invention, a restriction 30 is locatedwithin the outer-gas inlet 18 for increasing the gas velocity in thesheathing gas layer compared to the gas velocity therein in the absenceof said restriction.

In this embodiment restriction 30 is a nozzle formed within-theouter-gas inlet 18. The nozzle 30 has the effect of increasing thevelocity of the spiral gas flow and this serves to “stiffen” thesheathing gas layer upon exit from annular gap 22 and thus betterconfines a microwave induced plasma than would a typical torcharrangement that is used for ICP spectrometry.

One way of creating the nozzle 30 is to mould it directly as part of thegas inlet 18. As the microwave induced plasma torch 10 is typicallyconstructed of quartz which is a relatively difficult material to mouldwith accuracy, the nozzle may be formed by reducing the quartz outer-gasinlet 18 onto a piece of tungsten wire of appropriate diameter toachieve the quite close tolerancing that is required in the creation ofthe nozzle 30. An alternative approach is to machine the nozzle 30 as aseparate component which is either inserted and sealed into the gasinlet tube 18 or replaces the gas inlet tube 18. A third and convenientalternative is to fill part or all of the length of the outer-gas inlettube 18 with a potting material such as an epoxy resin 32 (see FIG. 2B.FIG. 2A shows the initial outer-gas inlet tube 18), curing the pottingmaterial 32 and then machining the nozzle 30 in the cured material 32(see FIG. 2C). This approach has proven to be simple and effective. Italso eliminates the need for dimensional accuracy in the quartz inlettubing 18.

Where a nozzle 30 is formed as a simple restriction, for a third gasflow of 15 litres per minute the preferred throat diameter of the nozzleis between 0.9 and 1.3 mm, although it is to be understood thatdifferent gas flow rates or different nozzle designs can result indifferent throat diameters. Typical pressure drops are in the range 50to 200 kPa.

Typically in a torch for radio frequency ICP spectrometry, the end 34 ofthe inner (sample injection) tube 16 is spaced back from the end 35 ofthe intermediate tube 14 to increase its separation from the plasma andthus reduce the temperature at its end 34. This reduction in temperatureboth reduces the risk of melting the inner tube 16 and reduces thelikelihood of premature evaporation of sample which would have theeffect of depositing the dissolved solids near the tube end 34 thusblocking the sample injection tube. However, in a preferred feature ofthe present invention, the end 34 is extended to be substantially level(for example within 2 mm) with the end 35 of intermediate tube 14. Thisimproves the injection of sample into a microwave induced plasma, whichinjection is more difficult than for a radio frequency ICP. The outletdiameter at end 34 for a sample gas flow of about 1 litre per minute ispreferably between 0.9 and 1.4 mm.

A further feature of the invention, which assists in preventing blockagein proximity to end 34 of inner tube 16, particularly if that end 34 issubstantially level with the end 35 of intermediate tube 14, is toassociate a heating means 36 with a section 38 of the inlet 26 for theinner tube 16. The tube section 38 may be constructed from a piece ofchemically and thermally resistant tube such as for example a quartz orglass tube having a resistance wire wound around the outside and hightemperature insulation covering the resistance wire and the tube. Thewire is heated by passing an electrical current through it and thesample is heated as it passes through the tube section 38 from one endto the other. As a non-limiting example of typical dimensions thefollowing arrangement has been found to be effective. A quartz tube 38of 9 mm inner diameter×11 mm outer diameter×150 mm long with the middle80 mm wound with 25 turns of flat nichrome wire 1.6 mm wide by 0.2 mmthick. The unheated ends of this quartz tube 38 are present to ensurethat the ends where hose connection is made remain cool. The coilresistance was 4 ohms and was heated using a 12 volt AC power supplythus delivering 36 watts. The whole assembly was enclosed in a block offibrous ceramic insulation 20 mm×20 mm×90 mm outside dimensions. It isto be understood however that many other geometries could be effectivewithout departing from the scope of the present invention.

It is to be understood that the invention includes a torch 10 (which maybe modified for ICP) having a heater 36 but which does not include arestriction 30 for increasing the downstream gas velocity. Such a torch10 with a heater 36 may be used for MIP or ICP spectroscopy.

As an indication of the effectiveness of the heater 36, the torch 10 wasfirst run with the tube section 38 in place but with the heating coilunenergised. Seawater with 3.5% total dissolved solids (TDS) wasintroduced and degrading sensitivity was observed within 1 minute of thestart of introduction of the sample. Signal degradation progressed untiltotal blockage occurred approximately 10 minutes after the start ofintroduction of the sample. The torch was then cleaned and theexperiment repeated but with the heating coil 36 energised. This time,no indication of blockage was observed after 15 minutes continuousintroduction of the sample, and when the torch was subsequentlydisassembled and examined, there was no sign of any deposit near the tipend 34 of the injector 16. A sample containing 10% TDS was thenintroduced continuously for 20 minutes with no sign of blockage.

The invention described herein is susceptible to variations,modifications and/or additions other than those specifically describedand it is to be understood that the invention includes all suchvariations, modifications and/or additions which fall within the scopeof the following claims.

1. A torch for plasma spectrochemical analysis including an outer tube,an intermediate tube and an inner tube, the inner tube beingsubstantially coaxially located within the intermediate tube forinjecting a first gas flow for carrying a sample for analysis into aplasma produced in the torch, an intermediate-gas inlet leading into theintermediate tube for admitting a second gas flow into the space betweenthe inner tube and the intermediate tube for controlling the axialposition of the plasma produced in the torch, an outer-gas inlet leadinginto the outer tube for supplying a third gas flow between the outertube and the intermediate tube for providing a sheathing gas layer forthe plasma produced in the torch, wherein the outer-gas inlet is offsetfrom a central axis of the torch to impart a spiral flow to the suppliedthird gas as it moves along the torch to provide the sheathing gaslayer, and means associated with the outer-gas inlet for increasing thegas velocity in the sheathing gas compared to the gas velocity upstreamof said means to thereby increase the confining force of the sheathinggas layer on the plasma.
 2. A torch as claimed in claim 1, wherein themeans associated with the outer-gas inlet is a restriction within theinlet.
 3. 4. A torch as claimed in claim 3, wherein the nozzle is formedin situ from a cured potting material within the outer-gas inlet.
 5. Atorch as claimed in claim 1, wherein the means associated with theouter-gas inlet is such as to, in use, cause a relatively high increasein the gas velocity.
 6. A torch as claimed in claim 1, wherein theintermediate and inner tubes terminate at respective ends within theouter tube, and wherein the ends of the intermediate and inner tubes aresubstantially level, for example within about 2 mm.
 7. A torch asclaimed in claim 6, wherein the inner tube has an outlet that is ofreduced diameter compared to an inlet end of the inner tube.
 8. A torchas claimed in claim 1, wherein the inner tube includes an inlet section,and a heating means is associated with said inlet section for heating anaerosol passing through that section to substantially completelyevaporate liquid from the aerosol, the section of the inner tube beingspaced from the outlet for the liquid to be substantially completelyevaporated before the aerosol reaches the proximity of the outlet.
 9. Atorch as claimed in claim 8, wherein the heating means is an electricalresistance heater.
 10. A torch as claimed in claim 9, wherein theelectrical resistance heater is provided by an electrical coil aroundthe inlet section.
 11. A torch for plasma spectrochemical analysisincluding an outer tube, an intermediate tube and an inner tube, theinner tube being substantially coaxially located within the intermediatetube for carrying a first gas flow for conveying an aerosol of anebulised sample liquid for injection through an outlet thereof into aplasma formed in the torch, an intermediate-gas inlet leading into theintermediate tube for admitting a second gas flow into the space betweenthe inner tube and the intermediate tube for controlling the axialposition of a plasma produced in the torch, an outer-gas inletleading,into the outer tube for supplying a third gas flow between theouter tube and the intermediate tube for providing a sheathing gas layerfor a plasma produced in the torch, wherein the outer-gas inlet isoffset from a central axis of the torch to impart a spiral flow to thesupplied third gas as it moves along the torch to provide the sheathinggas layer, and a heating means associated with a section of the innertube for heating an aerosol passing through that section tosubstantially completely evaporate liquid from the aerosol, the sectionof the inner tube being spaced from the outlet of the. inner tube forthe liquid to be substantially completely evaporated before the aerosolreaches the proximity of the outlet.
 12. A torch as claimed in claim 11,wherein the heating means is an electrical resistance heater.
 13. Atorch as claimed in claim 12, wherein the electrical resistance heateris provided by an electrical coil round the inlet section.
 14. Amicrowave induced plasma spectrochemical analysis system including atorch as claimed in claim 1, a gas supply for supplying a plasma supportgas to the outer-gas inlet of the torch, wherein the gas supply suppliesthe plasma support gas at a substantially constant pressure, whereby theflow rate of the plasma support gas into the torch is regulated by themeans associated with the outer-gas inlet for increasing the gasvelocity in the sheathing gas layer.